Origen

Silver that is not polished will tarnish, losing its lustre. The word goes back to French terne ‘dark, dull’. The metaphorical use, ‘to make less valuable or respected’, has been established since the 17th century. In 1805, on the eve of the Battle of Trafalgar, Lord Nelson prayed that God might grant them ‘a great and glorious victory; and may no misconduct in anyone tarnish it’.